The present invention relates to an apparatus and a method for alarming decrease in tire air-pressure. More particularly, it relates to an apparatus and a method for alarming decrease in tire air-pressure which is capable of detecting decompression of a tire even when performing high speed running with tires such as fuel-sparing tires which are soft and highly flat.
It is conventionally known for a system for alarming decrease in tire air-pressure (DWS) for detecting decompression of a tire based on rotational (wheel speed) information of four wheels. Such a system is based on the theory that a rotational velocity or rotational angular velocity of a decompressed tire is increased compared to other normal tires since its outer diameter (dynamic load radius of the tire) is decreased in contrast to tires of normal internal pressure. In a method for detecting decrease in internal pressure on the basis of a relative difference of rotational angular velocities of tires (refer to Japanese Unexamined Patent Publication No. 305011/1988), a judged value DEL is defined as follows:
DEL={(F1+F4)/2xe2x88x92(F2+F3)/2}/{(F1+F2+F3+F4)/4}xc3x97100(%) 
Here, F1 to F4 indicate rotational angular velocities of a front left tire, a front right tire, a rear left tire and a rear right tire, respectively.
Upon calculation of this DEL value, required correction such as cornering correction or reject of unnecessary data is performed, and it is determined that decompression of a tire has occurred in case the obtained value exceeds a preliminarily set threshold.
However, since centrifugal force acts on tires during running, tires are apt to be deflected outwardly. It might thus happen in conventional methods that a deflated tire cannot be detected since a tire diameter does not vary to a large extent owing to the effect of centrifugal force even though the tire is actually decompressed.
Such cases in which deflation of a tire cannot be detected might increasingly happen when performing high-speed running at approximately 150 km/h using tires of fuel-sparing design. Such tires of fuel-sparing design are of soft and highly flat structure and are especially apt to deflect outwardly since lockup force with respect to centrifugal force will be further weakened in case the internal pressure is decreased. With this reason, deflation can hardly be detected with a conventional apparatus for alarming decrease in tire air-pressure when performing high-speed running at approximately 150 km/h with such tires of fuel-sparing design.
The present invention has been made in view of the above facts, and it is an object thereof to provide an apparatus and a method for alarming decrease in tire air-pressure with which it is possible to detect decompression of a tire also when the vehicle is performing high-speed running.
In accordance with the present invention, there is provided an apparatus for alarming decrease in tire air-pressure which detects the decrease in internal pressure of the tire based on wheel speed information obtained from wheels of a vehicle and accordingly alarms a driver, comprising: a wheel speed information detecting means for detecting wheel speed information of the respective tires, a memory means for storing the wheel speed information of the respective tires, a calculating means for calculating a judged value based on a vehicle velocity and wheel speed information of the respective tires, and a judging means for determining a decrease in tire air-pressure based on correlation of the vehicle velocity and an outer diameter of the tire at the time of performing high-speed running.
In accordance with the present invention, there is also provided a method for alarming decrease in tire air-pressure which detects the decrease in internal pressure of the tire based on wheel speed information obtained from wheels of a vehicle and accordingly alarms a driver, comprising the steps of: detecting wheel speed information of the respective tires, storing the wheel speed information of the respective tires, calculating a judged value based on a vehicle velocity and wheel speed information of the respective tires, and determining a decrease in tire air-pressure based on correlation of the vehicle velocity and an outer diameter of the tire at the time of performing high-speed running.